This invention relates to a method of tempering a glass sheet for use, for example, as an automobile side or rear window by heating the glass sheet to a temperature above the strain point and quenching the heated glass sheet by directing cooling air jets onto both surfaces of the glass sheet.
The quenching of the glass sheet produces a center-to-surface temperature gradient through the thickness of the glass sheet and results in that permanent compressive stresses are produced in the surface layers of the glass sheet, with compensating tensile stresses in the center of the glass thickness. The tempered glass sheet is expected to break up into small and not very dangerous particles of glass when it is fractured by accident.
As to tempered glass sheets for use as automobile side or rear windows, there are official regulations which specify the manner of fragmentation of the tempered glass sheets. Such regulations commonly require that fracture of a tempered glass sheet should not produce dangerously large or elongated particles of glass. For example, British and European Economic Community (EEC) standards basically prohibit the presence of particles longer than 60 mm in which the length is not less than four times the width. Such particles are referred to as "splines". Besides, the same standards specify that the number of particles included in any 50 mm.times.50 mm square traced on the glass sheet (except in specified marginal areas and a specified circular area around the point of impact) should be within a limited range, such as from 60 to 400, and further specify a maximum permissible area of each particle, such as 300 mm.sup.2.
In the recent automobile industry a matter of important concern is reducing the vehicle weight. Accordingly there is a growing demand for tempered glass sheets of reduced thickness for use as side and rear windows. However, glass sheets less than about 3 mm in thickness are difficult to temper by the conventional air quenching method as so to comply with the aforementioned standards, primarily because of difficulty in creating and maintaining a suitable gradient of temperature in the thickness direction of the thin glass sheets during the quenching process.
With a view to satisfactorily tempering relatively thin glass sheets less than about 3 mm in thickness by air quenching, some proposals have been made for enhancement of the cooling efficiency. U.S. Pat. No. 4,578,102 proposes directing jets of a mixture of air and atomized water onto the heated glass surfaces by means of Laval nozzles. Air is supplied to the Laval nozzles at such a pressure that the jet velocity at the exit of each nozzle becomes at least sonic, while water is introduced from a radial direction into the constricted throat section of each nozzle to accomplish atomization of water and mixing of the atomized water with air within the divergent cone section of the nozzle. The mixture of air and atomized water has a higher specific heat than air alone. It is intended to rapidly extract heat from the glass sheet surfaces by using such high-velocity and high-specific heat two-phase jets. However, from a practical point of view this method is rather inconvenient and has some disadvantages. First, the necessity of using water besides air offers complicacy. Besides, very high precision of the equipment is required for complete atomization of water into a fine mist by using Laval nozzles and for thorough mixing of the atomized water with air during the transfer of the two fluids from the nozzle throat to the nozzle exit. Naturally a heavy cost is entailed. Furthermore, the relative pressure of air supplied to the nozzles must be at least 0.91 bar (about 0.93 kg/cm.sup.2) in order to enable the jet velocity at the exit of the nozzles to be sonic. Despite the complicacy of the equipment and operation, still it is difficult to eliminate the possibility of relatively large droplets of water hitting the heated glass sheet to cause the glass to break.
Japanese patent application primary publication No. 60-145921 relates to tempering of a glass sheet by directing jets of air from quenching nozzles onto the heated glass sheet surfaces, and proposes to determine the air pressure and the nozzle configuration such that the maximum drop of the cooling air pressure takes place at the exit of each nozzle whereby the air jet velocity at the nozzle exit becomes sonic. The quenching nozzles used in this method are straight nozzles narrowed at the exit so as to form a small orifice, and the pressure of cooling air supplied to the nozzles is at least 0.9 bar (about 0.92 kg/cm.sup.2) by gauge pressure. A disadvantage of this method is that fluctuations of the air supply pressure in the quenching equipment are liable to be transmitted to the glass sheet surfaces so that the heated glass sheet under quenching is liable to be distorted, particularly when the glass thickness is less than 3 mm. Besides, in this method it will be necessary to give very careful consideration to the arrangement of the quenching nozzles on a plane parallel to the glass sheet.